The Brown Rat (Rattus norvegicus) is one of the most well-known and common rats, and also one of the largest. Traditionally the dominant rodent of Europe and thought to have originated in northern China, the Brown Rat has spread to all continents, and is a common pest wherever humans live, particularly in urban areas and degraded environments. It has been given a number of different names, including Ship Rat, Norway Rat, Wharf Rat and Common Rat.

The Brown Rat is a true omnivore and will consume almost anything, but prefers grains, and eats up to a third of bodyweight a day. Brown Rats are known to catch fish or small rodents. They are usually active at night and are good swimmers, but poor climbers.

The fur is coarse and usually brown or dark grey, the underparts are lighter grey or brown. Length can be up to 40 cm, although 25 cm is more common, with the tail a further 15 cm or so (less than half the body length). Adult body weight averages 320 g in males and about 200 g in females, but a very large individual can reach 500 g. Brown Rats have acute hearing and are sensitive to ultrasound. Average heart rate is 300-400 bpm, with a respiratory rate of around 100 per minute. Their vision is very poor and they are unable to detect colour and are blind to long-wave light.

The Brown Rat can breed throughout the year if conditions are suitable, a female producing up to eight litters a year. The gestation period is only 21-23 days and litters can number up to fourteen, although seven is common. Lifespan is up to three years, although most barely manage one—a mortality rate of 95% is estimated: predators, intraspecific conflict, and cannibalism are major causes. Brown Rats live in large hierarchical groups, either in burrows or subsurface places such as sewers and cellars.

Selective breeding of the Brown Rat has produced the albino laboratory rat.

Brown Rats make excellent pets and are the focus of attention at shows of fancy rats. Brown Rats are clean, social and intelligent pets.